Detection of ultrasonic signals can be useful in various situations. A common example is the detection of leaks in equipment using compressed air or other gases, such as lines and hoses used to supply compressed air to pneumatic power tools and equipment in industrial settings. Leaks can occur as a result of cuts or cracks in the lines, and ultrasonic waves are generated as compressed air escapes through a cut or crack. For reasons of safety and economy, it is essential that compressed air lines be continuously monitored for defects.
A basic knowledge of sound wave principles is necessary for the understanding of the process of detecting leaks by analyzing ultrasonic waves. The audible sound wave spectrum covers wave frequencies beginning at .apprxeq.20 Hz and ending at .apprxeq.20 kHz. Waves with higher frequencies are considered ultrasonic. Sound emanating from a source will typically be a composite of waves over a wide frequency range. As a wave travels farther away from its source it attenuates or flattens out, and it is well known in the art that higher frequency waves attenuate at faster rates than low frequency waves.
Numerous prior art devices have been developed to detect leaks. U.S. Pat. No. 3,462,240 discloses an instrument which detects and amplifies acoustic vibrations caused by fluid flowing through a hole in a pipeline. It divides the vibrations into high and low frequencies, and produces an alarm when one signal band generates a higher intensity than the other signal band.
U.S. Pat. No. 5,040,409 provides for an apparatus and method for detecting when a pipe break in a sprinkler system occurs. This device also generates an alarm when one frequency band produces a higher intensity reading than another frequency band.
U.S. Pat. No. 4,635,042 discloses a device for locating the sources of ultrasonic sound within a narrow frequency band indigenous to vacuum leaks through the utilization of a hand-held probe containing a transducer exhibiting a sensitivity peak within the band, at about 40 kHz, and at least one other sensitivity peak outside the narrow band.
U.S. Pat. Nos. 4,289,019, 4,785,659, 4,858,462 and 4,958,296 also propose devices for detecting leaks in pipes or hoses.
While the foregoing devices are effective in detecting leaks and are in widespread use, they are subject to high levels of interference by originating sources other than the leak and often give false indications of a leak in response to the detection of ultrasonic signals from sources other than gas leaks that are commonly present in industrial environments. Moreover, when two ultrasonic leaks exist simultaneously, previously known devices are often incapable of distinguishing between leaks within the desired detection zone and leaks outside the desired detection zone.
Accordingly, it is desirable to have ultrasonic leak detection apparatus and methods that can distinguish between simultaneous leaks within a selected detection zone and outside the zone, and that can eliminate additional interference to indicate only sources located within the selected zone.